Materials composed of cemented hard particles are technologically and commercially important. Cemented hard particles include a discontinuous dispersed phase of hard metallic (i.e., metal-containing) and/or ceramic particles embedded in a continuous metallic binder phase. Many such materials possess unique combinations of abrasion and wear resistance, strength, and fracture toughness.
Terms used herein have the following meanings. “Strength” is the stress at which a material ruptures or fails. “Fracture toughness” is the ability of a material to absorb energy and deform plastically before fracturing. “Toughness” is proportional to the area under the stress-strain curve from the origin to the breaking point. See McGraw Hill Dictionary of Scientific and Technical Terms (5th ed. 1994). “Wear resistance” is the ability of a material to withstand damage to its surface. “Wear” generally involves progressive loss of material due to a relative motion between a material and a contacting surface or substance. See Metals Handbook Desk Edition (2d ed. 1998).
The dispersed hard particle phase typically includes grains of, for example, one or more of a carbide, a nitride, a boride, a silicide, an oxide, and solid solutions of any of these types of compounds. Hard particles commonly used in cemented hard particle materials are metal carbides such as tungsten carbide and, thus, these materials are often referred to generically as “cemented carbides.” The continuous binder phase, which binds or “cements” the hard particles together, generally includes, for example, at least one of cobalt, cobalt alloy, nickel, nickel alloy, iron and iron alloy. Additionally, alloying elements such as, for example, chromium, molybdenum, ruthenium, boron, tungsten, tantalum, titanium, and niobium may be included in the binder phase to enhance particular properties. The various commercially available cemented carbide grades differ in terms of at least one property such as, for example, composition, grain size, or volume fractions of the discontinuous and/or continuous phases.
For certain applications parts formed from cemented hard particles may need to be attached to parts formed of different materials such as, for example, steels, nonferrous metallic alloys, and plastics. Techniques that have been used to attach such parts include metallurgical techniques such as, for example, brazing, welding, and soldering, and mechanical techniques such as, for example, press or shrink fitting, application of epoxy and other adhesives, and mating of mechanical features such as threaded coupling and keyway arrangements.
Problems are encountered when attaching cemented hard particle parts to parts formed of steels or nonferrous alloys using conventional metallurgical or mechanical techniques. The difference in coefficient of thermal expansion (CTE) between cemented carbide materials and most steels (as well as most nonferrous alloys) is significant. For example, the CTE of steel ranges from about 10×10−6 in/in/° K to 15×10−6 in/in/° K, which is about twice the range of about 5×10−6 in/in/° K to 7×10−6 in/in/° K CTE for a cemented carbide. The CTE of certain nonferrous alloys exceeds that of steel, resulting in an even more significant CTE mismatch. If metallurgical bonding techniques such as brazing or welding are employed to attach a cemented carbide part to a steel part, for example, enormous stresses may develop at the interface between the parts during cooling due to differences in rates of part contraction. These stresses often result in the development of cracks at and near the interface of the parts. These defects weaken the bond between the cemented hard particle region and the metal or metallic region, and also the attached regions of the parts themselves.
In general, it is usually not practical to mechanically attach cemented hard particle parts to steel or other metallic parts using threads, keyways or other mechanical features because the fracture toughness of cemented carbides is low relative to steel and other metals and metallic alloys. Moreover, cemented carbides, for example, are highly notch-sensitive and susceptible to premature crack formation at sharp corners. Comers are difficult to avoid including in parts when designing mechanical features such as threads and keyways on the parts. Thus, the cemented hard particle parts can prematurely fracture in the areas incorporating the mechanical features.
The technique described in U.S. Pat. No. 5,359,772 to Carlsson et al. attempts to overcome certain difficulties encountered in forming composite articles having a cemented carbide region attached to a metal region. Carlsson teaches a technique of spin-casting iron onto pre-formed cemented carbide rings. Carlsson asserts that the technique forms a “metallurgical bond” between the iron and the cemented carbide. The composition of the cast iron in Carlsson must be carefully controlled such that a portion of the austenite forms bainite in order to relieve the stresses caused by differential shrinkage between the cemented carbide and the cast iron during cooling from the casting temperature. However, this transition occurs during a heat treating step after the composite is formed, to relieve stress that already exists. Thus, the bond formed between the cast iron and the cemented carbide in the method of Carlsson may already suffer from stress damage. Further, a bonding technique as described in Carlsson has limited utility and will only potentially be effective when using spin casting and cast iron, and would not be effective with other metals or metal alloys.
The difficulties associated with the attachment of cemented hard particle parts to parts of dissimilar materials, and particularly metallic parts, have posed substantial challenges to design engineers and have limited the applications for cemented hard particle parts. As such, there is a need for improved cemented hard particle-metallic and related materials, methods, and designs.